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Python Dictionaries

Introduction to dictionaries

In Python, dictionaries are a fundamental data structure used to store collections of data. Unlike lists or tuples, which store elements in a specific order, dictionaries organize data using key-value pairs. This makes them incredibly useful for situations where you need to quickly access and retrieve specific pieces of information.


Why use dictionaries?

Dictionaries excel at efficiently storing and retrieving data based on a unique identifier, the key. This efficiency comes from a clever internal structure that allows Python to quickly locate the value associated with a particular key.

Here's an analogy: Imagine a dictionary (like a physical book) with entries containing words (keys) and their definitions (values). You can instantly find the definition of a word (value) by looking up its word (key) in the index (similar to how dictionaries work internally).


Example dictionary

my_dictionary = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}


Key characteristics of dictionaries

  • Key-value pairs: Each entry in a dictionary consists of a key and a corresponding value. Keys act as unique identifiers that help you access the associated value.
  • Keys: Must be unique within a dictionary. No two keys can be the same. Must be immutable data types like strings, numbers, or tuples. This ensures they can't be changed accidentally, maintaining the integrity of the key-value association.
  • Values: Can be any data type in Python, including strings, numbers, lists, dictionaries (nested dictionaries), or even custom objects. This flexibility allows you to store a wide variety of information.
  • Unordered: Unlike lists or tuples, dictionaries do not maintain a specific order of their elements. The order in which key-value pairs are added or displayed may not be the same. This is because dictionaries prioritize efficient retrieval by key over maintaining a sequence.
  • Mutable: Once created, you can modify the values associated with existing keys in a dictionary. This allows you to update or change the information stored. However, you cannot change the keys themselves due to the uniqueness requirement.

In summary:

Dictionaries are powerful tools for storing and managing data in Python. They provide efficient key-based access, making them ideal for various use cases like phonebooks, configuration settings, user profiles, data analysis (counting word frequencies), and many more!

It's time to take a quiz!

Test your knowledge and see what you've just learned.

What is a dictionary in Python?

A type of list
An immutable collection of data
A data structure that stores elements in order
A collection of key-value pairs
Check Answer

In a dictionary, what are the key-value pairs used for?

To store and retrieve data efficiently
To keep data in a specific order
To represent only numbers
To create a list of items
Check Answer

Which of the following is true about keys in a dictionary?

Keys must be unique
Keys can only be strings
Keys can be duplicated
Keys must be mutable
Check Answer

What does it mean that a dictionary is mutable?

You can change the keys
You can change the values associated with keys
The order of elements is fixed
You can only add new keys
Check Answer

How do you access the value associated with a key in a dictionary?

Using the get() method only
Directly referencing the key
Using square brackets, e.g., my_dict["key"]
Using parentheses, e.g., my_dict("key")
Check Answer

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Built-in Set methods

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Creating Dictionaries